Physiology of Digestion Flashcards
What are the three things that carbohydrates are digested to?
Polysaccharides
Oligosaccharides
Monosaccharides
What must carbohydrates be converted to for absorption?
monosaccharides
Describe Polysaccharides?
Starch - two kinds
Amylose is a linear molecule where the individual glucose molecules are held together by alpha 1,4 bonds
Amylopectin is branched chain and linked by alpha 1,6 bonds
Glycogen (in plants) - branched chain α-1,4 and
α-1,6 linkages
Describe Oligosaccharides?
Sucrose = Glucose + Fructose, α-1,2 linkages Lactose = Glucose + Galactose ß-1,4 linkages
Describe monosaccharides?
Glucose
Fructose
What enzyme is involved in carbohydrate digestion?
α-Amylase
What are the steps of carbohydrate digestion?
Intraluminal hydrolysis = starch to Oligosaccharides Membrane digestion (at brush border) = Oligosaccharides to monosaccharides
Describe alpha amylase?
Endoenzyme
breaks down linear internal α-1,4 linkages but not terminal α-1,4 linkages. Hence, no production of glucose
cannot cleave α-1,6 linkages at branch points (in amylopectin) or α-1,4 linkages adjacent to branch points
products are thus linear glucose oligomers (maltotriose, maltose) and α-limit dextrins
What is the role of Oligosaccharides?
Oligosaccharidases are integral membrane proteins with a catalytic domain that faces the lumen of the GI tract
- Lactase has only one substrate – breaks down lactose to glucose and galactose
- All other oligosaccharidases cleave the terminal α-1,4 linkages of maltose, maltotriose and α-limit dextrins (to yield glucose)
What is Sucrase responsible for?
for hydrolysing sucrose to glucose and fructose
What is unique about Isomaltase?
enzyme that can split the branching α-1,6 linkages of α-limit dextrins
What is the role of maltase?
can degrade the α-1,4 linkages in straight chain oligomers up to nine monomers in length
What can lactose intolerance come from?
Primary lactase deficiency (primary hypolactasia) – due to lack of the lactase persistence (LP) allele – most common cause world wide
Secondary lactase deficiency – caused by damage to/ infection of/ the proximal small intestine
Congenital lactase deficiency – rare autosomal recessive disease – no ability to digest lactose from birth
What are the consequences of lactose consumption in lactose intolerance?
If lactose is delivered to the colon from the ileum colonic, microflora produce:
short-chain fatty acids (which can be absorbed)
hydrogen (H2 - which can be detected in the breath of lactase deficient individuals following a lactose challenge)
carbon dioxide
methane
These by products produce: bloating, abdominal pain, flatulence
What does undigestid lactose cause?
acidification of the colon
an increased osmotic load – loose stools and diarrhoea
Describe the absorption of the Final Products of Carbohydrate Digestion: Glucose, Galactose and Fructose?
Occurs in the duodenum and jejunum
Is a two step process involving entry and exit from the enterocytes via the apical and basolateral membranes, respectively
Glucose and galactose are absorbed by secondary active transport mediated by SGLT1; fructose by facilitated diffusion mediated by GLUT5.
Exit for all monosaccharides is mediated by facilitated diffusion by GLUT2
What are the rules for SGLT1 transport?
A hexose in the D-conformation
One that can form a pyranose ring
Describe the mode of operation of SGLT1?
- 2 Na+ binds:
- Affinity for glucose increases, glucose binds:
- Na+ and glucose translocate from extracellular to intracellular:
- 2 Na+ dissociate, affinity for glucose falls:
- Glucose dissociates:
- Cycle is repeated
How must a protien be absorped?
digested to oligopeptides and amino acids
How are protiens converted to peptides?
luminal enzymes
How many proteins digestion pathways are there?
4 - LOOK AT THE POWERPOINT AND LEARN THEM
Describe digestion in the stomach?
HCl begins to denature proteins
Pepsin cleaves proteins into peptides
- has pH optimum of 1.8 to 3.5, inactivated at alkaline pH
- is an endopeptidase with preference for bonds between aromatic and larger neutral amino acids
Describe digestion in the duodenum?
The five pancreatic proteases are secreted as proenzymes from the exocrine pancreas and converted to active form in the duodenum.
They function as either endopeptidases, or exopeptidases
Describe the 5 pancreatic proteases?
Trypsin - Endopeptidase- product: Oligopeptides (2-6 amino acids)
Chymotrypsin - Endopeptidase - product: Oligopeptides (2-6 amino acids)
Elastase - Endopeptidase - product: Oligopeptides (2-6 amino acids)
Procaroxypeptidase A - Exopeptidase- product: Single amino acids
Procarboxypeptidase B - Exopeptidase - product: Single amino acids
